1. Field of the Invention
This invention relates generally to an information processing apparatus using a multiple-processor system, and more particularly to a cache memory control method in such information processing apparatus.
2. Description of the Related Art
Heretofore, the store-through system and the store-in system are known as cache memory control methods in an information processing apparatus. The store-in system and the store-through system are also called "write-back or write-in system" and "write-through system", respectively. A cache memory is also called "buffer storage". The store-through system is a method for progressively updating both a cache memory and a main storage when a request to write into the main storage is issued. The store-in system is a control method for updating only the cache, when a request to write is issued, and thereafter collectively writing back to the main storage. The store-through system and the store-in system are exemplified by Computing Surveys Vol. 14 No. 3, September 1982, pages 500-502. When an information processing apparatus uses a cache memory control method, whether to use the store-through system or to use the store-in system is a significant factor of a logical system in the information processing apparatus and has a great influence on its performance.
Generally, the store-through system is disadvantageous, compared to the store-in system, in that it is complex to control so that the quantity of hardware increases. But since it is capable of reducing the frequency of transfer with the main storage, the store-through system is suitable for, for example, closely joined multiple processors which share one main storage with another processor.
Here assume that in a multiple-processor system in which one main storage is shared by a plurality of processors, an area in the main storage is used as a common data area, and the individual processors are equipped with respective cache memories using the store-in system.
In this case, if the first processor reads from a cache memory a block belonging to this common area and writes the block back in the cache memory, the latest data would exist only in the cache memory of the first processor. And, the data of the main storage will be old data until it is written.
At that time, for the second processor to access the data belonging to this block of the common area, the second processor must read the block to access its own cache memory after the entire data of the block which are stored in the first cache memory have been written back to the main storage. To write this cache memory data back to the main storage is overhead which is inevitable with the store-in system.
Like the common data area, the write-back will frequently occur with the data which are shared by the multiple processors. Therefore, the overhead will increase to lower the performance of the multiple-processor system.
Then assume that in a multiple-processor system in which one main storage is shared by a plurality of processors, the individual processors are equipped with respective cache memories using the store-through system.
In this case, it is sufficient that a peculiar data area dedicated for use by a particular job or task exists only in the cache memory in a specified processor while the job runs by the processor. However, since the corresponding main storage data are updated every time writing into this peculiar data area occurs, the frequency of the main storage transfer will increase to lower the performance of the multiple-processor system.
Therefore, either of the store-in and store-through systems is unsuitable as a control method under certain situations to thereby lower the performance of the multiple-processor system.